Naturally occurring and synthetic zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Certain zeolites are ordered porous crystalline aluminosilicates having definite crystalline structure as determined by X-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by structure of the crystal. The zeolites are referred to as "molecular sieves" because the uniform pore size of a zeolite material may allow it to selectively adsorb molecules of certain dimensions and shapes.
By way of background, one authority has described the zeolites structurally, as "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms. Furthermore, the same authority indicates that zeolites may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
In the empirical formula, x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated D. M. Breck, ZEOLITE MOLECULAR SIEVES, John Wiley & Sons, New York p. 5 (1974). In the empirical formula, the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. M was described therein to be sodium, potassium, magnesium, calcium, strontium and/or barium, which complete the electrovalence makeup of the empirical formula.
The prior art describes a variety of synthetic zeolites. These zeolites have come to be designated by letter or other convenient symbols, as illustrated by the zeolite. The silicon/aluminum atomic ratio of a given zeolite is often variable. Moreover, in some zeolites, the upper limit of the silicon/aluminum atomic ratio is unbounded. ZSM-5 is one such example wherein the silicon/aluminum atomic ratio is at least 2.5 and up to infinity. U.S. Pat. No. 3,941,871, reissued as U.S. Pat. No. Re 29,948, discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum and exhibiting the X-ray diffraction pattern characteristic of ZSM-5.
Various patents describe inclusion of elements other than silicon and aluminum in the preparation of zeolites. Cf. U.S. Pat. No. 3,530,064, U.S. Pat. Nos. 4,208,305 and 4,238,318 describe the preparation of silicates in the presence of iron. ZSM-5 is a member of a class of zeolites sometimes referred as medium pore zeolites. The pore sizes of medium pore zeolites range from about 5 to about 7 Angstroms.
Another class of zeolites sometimes referred to as large pore zeolites include inter alia naturally occurring faujasite, synthetic zeolites X, L, Y, and zeolite beta. These zeolites are characterized by pore sizes greater than those of the medium pore zeolites. The pore sizes of large pore zeolites are greater than about 7 Angstroms. Because of the larger pore sizes these latter zeolites may be less (molecule) shape selective. Zeolite beta is the subject of U.S. Pat. No. 3,308,069 and U.S. Pat. No. Re 28,341, each of which describes the X-ray diffraction of zeolite beta, which is incorporated by reference herein. Like ZSM-5, zeolite beta has been reported to have been made with various framework elements. Cf. M. Taramasso et al., "Molecular Sieve Borosilicates," Proceedings of the Fifth International Conference on Zeolites, Heyden & Son Ltd. 40-48 (1980), and copending application Ser. No. 836,210 filed Feb. 28, 1986, each of which is relied upon and incorporated by reference herein.